Investigating Interfacial Electron Transfer in Highly Efficient Porphyrin-Sensitized Solar Cells

The YD2-o-C8 complex based on zinc tetraphenylporphyrin (ZnTPP) is among the most efficient sensitizers for DSSCs to date, reaching over 12% in power conversion efficiency when paired with an organic co-sensitizer on a TiO2 surface. To understand the link between the structure and light-harvesting properties of ZnTPP dyes, four sensitizers based on ZnTPP are investigated, with a systematic addition of structural features found in YD2-o-C8. Density functional theory (DFT) and time-dependent DFT are utilized to obtain the ground and excited state electronic structure of each chromophore. Quantum dynamics simulations are also employed to investigate interfacial electron transfer between the electronically excited ZnTPP dyes and the TiO2 semiconductor. Overall, we find that substitution of the strongly coupled ethynyl bridged benzoic acid and diarylamine at the meso position of the porphyrin ring have the most significant impact on the absorption properties and IET efficiencies of Zn TPP based dyes. The addition of the alkoxy groups and long chain alkanes onto the meso substituted phenyl groups to produce YD2-o-C8 have little impact on the ground and excited state properties of the sensitizer. The structural modifications that do not impact the chromophore electronic structure are, however, likely to impact intermolecular interactions (chromophore-chromophore and chromophore-electrolyte). Overall, our results shed light on the origins of the sensitization efficiency of the YD2-o-C8 dye and support the conclusions of previous experimental and computation work on this system.